I have been complaining for years that US customers don't seem interested in having spot diagrams supplied for the equipment they are considering purchasing, and this paper has it all.

This is the kind of data US consumers should demand from suppliers.

I was an early champoin of the EdgeHD and remain convinced that it is one of the finest telescope I have ever owned. The quality is fantastic, and the visual off axis performance is as good as my 6" APO.

And now we have the data that shows exactly how good the performnace can be.

Of particular interest are the cutaway diagrams of the various desings that shows how rays comeing from the edge of the mirrors just miss the baffles. Many people struggle with how an SCT can loose aperture with back focus, and from these drawings, it is possible to see how moving the mirror forward only a few millimeters can cause the light cone to be cut off.

Anyway, an superb paper because of the technical content. I wish amateurs would insist on seeing spots for the equipment they are buying.

But because most do not truely understand how to read the spots, and because they often give the impression that a telescope does not perform as well as it does (you have to understand the distribion methods and density), I can understand why they have been reluctant to do so in the past.

But imagers want it more than ever, and we all should follow their example.

My hat is off to Celestron for both producing the perfect SCT, and for having the guts to publish the spots that show how excellent the design really is.

Yeah I'm still digesting it, lots of good info. I like how they go into the philosophy behind the design and their thinking behind it. They even produced a couple of CDK prototypes, before coming up the Edge.

Yes, this paper is to me ground breaking in that it is the first time I have ever seen spots actually published by Celestron.

And it confirms what another forum member told me a couple of weeks ago, which is that the formula for the EdgeHD scopes had changed (f/1.9 primary). When the design first came out, other members of the Cats and Casses forum swore that the mirror forumala had not changed.

I was convinced that it had though, because the optimal back focus positions had changed, and I did not think it would have been possible without changing something in the mirror forumla. I was told that I was wrong though, and a year later someone told me that the forumla had indeed changed.

And now the paper confirms this.

But this is an excellent paper and I am thrilled to see that Celestron put it out there.

And it clearly shows the huge improvement in off axis performance.

The spots even shows that a sharp eyed observer can indeed see the expanded fresnel pattern of a defocused star.

I had people I think not believe me when I would tell them that using a 22mm Nagler, I could see that stars at the outside were defocused enought to show a couple of rings, and I think they simply thought I was daffy.

But the spots show it clearly. The Standard SCT has a couple of wavelenghts of defocus at the edge of a low power eyepiece, and with modern wide field eyepeices, a sharp eyed observer can easily see this.

Celestron should be praised by all of us for finally matching Takahasi by publishing spot diagrams for at least some of its scopes.

Once again though, I am not confident that enough people out there understand how spots are generated and how to interpret them. For example the spot diagrams for coma far overstate how much you can see at the eyepiece because the density at the point is many dozens of times more intense as it is even part of the way out in the fan.

But the fan will show in a long exposure picture, but not visually.

With this informatoin, I hope that more people start to take an interest in spot diagrams, how to read them, and the value they bring to the consumer.

The spots may have things exaggerated some in terms of the full extent that light can be seen. It does however though, show how much off axis aberrations will affect limiting magnitude of stars. You can really only go deep right in the center with the normal SCT and the Meade's, but the Edge can do it way off axis as well. That light being spread out is going to push the stars below the background light threshold.

The Edge is also showing tighter on axis as well, but only by a little. I think it's the tight spots all over the field that are contributing to the feeling (visually) that the Edge's are so sharp and contrasty.

Pretty cool paper. And while I'm pretty pleased with my new 11" Edge HD, the one deficiency it has compared to my 11" NexStar GPS is that it doesn't have enough focus range. I only just discovered this a couple of days ago. I just bought a binoviewer and while it works great with my Denkmeier PowerXSwitch and the GPS, I can't reach focus with the reducer on the Edge HD. The mirror doesn't move nearly as far. I realize this is most likely due to the sweet spot for the Edge optics, but it's still disappointing, especially given the incredible results I'm getting with the binoviewer (should have bought one a LONG time ago!) and the NexStar GPS.

Pretty cool paper. And while I'm pretty pleased with my new 11" Edge HD, the one deficiency it has compared to my 11" NexStar GPS is that it doesn't have enough focus range. I only just discovered this a couple of days ago. I just bought a binoviewer and while it works great with my Denkmeier PowerXSwitch and the GPS, I can't reach focus with the reducer on the Edge HD. The mirror doesn't move nearly as far. I realize this is most likely due to the sweet spot for the Edge optics, but it's still disappointing, especially given the incredible results I'm getting with the binoviewer (should have bought one a LONG time ago!) and the NexStar GPS.

It's a pity they haven't produced a focal reducer for the 800 and 925. Still I don't understand why.

Explanation here, from the horse's mouth.In light of the scaling issues, logic would seem to dictate that C would have designed the FR for the 8 next, rather than the 11. However, they may make more money from 11 sales. Didn't think to ask the question when I had Kopit's ear.

Celestron was handing out copies of that white paper at ASAE. Fascinating read.

Still wish they'd produce a 6, but with production costs close to an 8 Edge, I understand why they don't - I don't think I'd be willing to spend ~$1K for a 6.

What I like about the white paper is the cut aways of both the scopes and the reducers. Even for non-experts on optics like me, it is easy to see why the reducers for the Edge HD are nothing like the old correctors for the standard SCTs and have clearly involved a lot of design and more difficult manufacturing and thus the price. Just those pictures have been the best explanation for the increased cost that I have seen (even though I hope that the cost comes down).

Yes I balked a bit at the price of the FR, but having got it, the price made some sense. Its a seriously heavy duty piece of metal & glass. I'm curious about its visual use, I've only managed to try it once and couldn't bring the scope to focus. However I had the big Baader Visual back on the end of it and the Baader 2" diagonal. I think the stock diagonal attached directly to the FR may do the trick.

Two things keep me from accepting such analyses. First, the standard professional scope type is the R/C, curved field and all. Yet, they use flat sensors and get scientifically-valid results from those instruments.

Second, is a whole host of images like this one and this one taken with a Meade 12" ACF bare (no reducer/flattener, etc.). I may be nuts, but I don't see any star bloat at the edges of those frames.

EDIT: Here's another one, taken with a roughly APS-H size sensor (bigger than APS-C, smaller than full-frame 35mm). Do you see signs of field curvature in that image?

Second, is a whole host of images ... taken with a Meade 12" ACF bare (no reducer/flattener, etc.).

+1.
Just check images posted to 'CCD Imaging & Processing' forum by Rick J. The practical impact of the field curvature of the R/C and the ACF on the quality of pretty pictures is insignificant.

Maybe I just figured out why?

The blur radii given at the edge of the f/10 coma-free scope in the Celestron white paper (which may or many not be the actual spot sizes of the Meade ACF scopes), are smaller than 2 arc seconds - the common limit of seeing for long-exposure photography. So, nearly all of the size of the smallest stars isn't coming from either diffraction or field curvature, it's coming from the atmosphere.

The blur radii given at the edge of the f/10 coma-free scope in the Celestron white paper (which may or many not be the actual spot sizes of the Meade ACF scopes), are smaller than 2 arc seconds - the common limit of seeing for long-exposure photography. So, nearly all of the size of the smallest stars isn't coming from either diffraction or field curvature, it's coming from the atmosphere.

CCD Inspector does show star bloat in ACF images. However you have to look really hard to find its effect in a pretty picture.

Two things keep me from accepting such analyses. First, the standard professional scope type is the R/C, curved field and all. Yet, they use flat sensors and get scientifically-valid results from those instruments.

Actually, this is an area (scientific work) where things are very different than with pretty pictures, and a lot of aberrations/problems have little/no impact. For example, photometry doesn't even require the stars to be in focus, or the tracking to even be perfectly accurate. Field curvature or other problems that spread out the star light will not have an impact other than reduced sensitivity.

Another is the R/C design as it relates to astrometry. The inherit aberration in an RC is astigmatism, however this has one nuance that means it will have no effect on use in measuring positions in that the star centroid is always in the same spot relative to the blur of the spot plot. That is the reason the RC design was chosen for this role. Field curvature will not affect that (other than reduced sensitivity, as I understand it).

As for how images look in scopes with field curvature, it's hard to tell how they were focused as well. Could be going for a midway point to try to average things out (little less focused at center, more at the edge). This is often what people suggest to do with standard SCT's and visual observing. I find it just looks "soft" everywhere.

Of course, combine that with wider seeing disks and more average light in the first ring in an exposure lasting several minutes and things are going to take on a larger size in terms of star appearance. I wouldn't say any of those images that I looked at have particularly tight stars. We don't know how they were processed of course.

I think the last image in the quoted white paper is sharper (the single frame of M51 in b&w). Or for example this one taken with a newtonian:

Two things keep me from accepting such analyses. First, the standard professional scope type is the R/C, curved field and all. Yet, they use flat sensors and get scientifically-valid results from those instruments.

Another is the R/C design as it relates to astrometry. The inherit aberration in an RC is astigmatism, however this has one nuance that means it will have no effect on use in measuring positions in that the star centroid is always in the same spot relative to the blur of the spot plot. That is the reason the RC design was chosen for this role. Field curvature will not affect that (other than reduced sensitivity, as I understand it).

As for how images look in scopes with field curvature, it's hard to tell how they were focused as well. Could be going for a midway point to try to average things out (little less focused at center, more at the edge). This is often what people suggest to do with standard SCT's and visual observing. I find it just looks "soft" everywhere.

R-C scopes have evolved over time. Starting in the 1960s with the invention of the Gascoigne corrector, which got rid of most of the astigmatism, then the addition of field-flattening optics, the professionals from the 1970s were no longer having big issues with field curvature in the way they did with the classic Schmidt camera (it's no fun bending glass plates).

If you have a look at the Sloan Digital Sky Survey Project Book, there's a section on the optical design of the 2.5 meter telescope, that discusses these matters and how they were dealt with.